Neptune will be NASA's next priority in the outer solar system

diciembre 15, 2018

Neptune will be NASA's next priority in the outer solar system https://danielmarin.naukas.com/files/2018/12/Captura-de-pantalla-66-1024x986.png

Uranus and Neptune are the least known planets of the solar system. Hence, NASA has spent years trying to outline the characteristics of a mission to study these bodies after 2030. In 2017 A studyfrom NASA concluded that it was necessary to study both worlds. The problem is that there is no money to launch two high-cost probes to the two planets, so there is a need to prioritize. One option is combine this mission with the visit to one or several objects of the Kuiper belt. Another is to explore one of the two planets. Until now, NASA considered that both worlds were equally interesting from a scientific point of view, but in a recent report by OPAG (Outer Planets Assessment Group) the agency has radically changed its mind. Now the priority is Neptune.

Neptune and Triton (NASA).

Why? Well because of Triton, the largest moon of Neptune. The decision does not stop being curious, because until recently it was Uranus who seemed to have won the battle. Uranus and Neptune are equally fascinating. Voyager 2 discovered that Neptune was more active than Uranus, but observations from the Hubble space telescope and ground instruments have shown that the latter planet can be as or more active than Neptune. On the other hand, the Uranus moons system is the worst known of the solar system, which compensated for the presence of Triton around Neptune. However, the proximity of Uranus was the main factor in their favor, since it allows flight times ostensibly less than those necessary to reach Neptune. Of the four proposed probes for the study of Uranus and Neptune of the OPAG, the Neptune orbiter was by far the most expensive option.

Proposed Neptune orbiter with propulsion stage SEP (NASA).

The four main OPAG options for the study of Uranus and Neptune. From left to right: Neptune orbiter with atmospheric probe and SEP stage, Uranus overflight probe with atmospheric probe, Uranus orbiter with atmospheric probe and Uranus orbiter without atmospheric probe (NASA).

But OPAG does not support any compromise solution that includes overflight probes to the two planets to visit other objects in the Kuiper belt. The only valid option for them is to send a mission of type flagship formed by a 7-ton orbiter equipped with an atmospheric probe that studies in detail one of the ice giants and its system of moons. And, since there is no money for two orbiters, you have to choose. Triton has decided the balance in favor of Neptune because the community of researchers dedicated to the ocean worlds of the solar system has identified it as one of its objectives. The largest moon of Neptune probably possesses an internal ocean - actually a mantle of liquid water - comparable to Pluto's. After all, Triton is a body of the Kuiper belt captured by Neptune. The ocean worlds - Europe, Ganymede, Callisto, Enceladus, Titan, etc. - are a priority for NASA because they are the bodies with the highest habitability potential of the solar system at this time. So, according to the OPAG, NASA's next flagship mission after Europe Clipper must be an orbiter to Neptune.

Ocean worlds of the solar system (confirmed and possible) (NASA).

32 overflights to study Triton in detail (NASA).

Atmospheric probe of the mission to the ice giants (NASA).

What's the rush? Well, because the OPAG wants to start developing a mission to Neptune before 2020 with the objective of taking advantage of the launch window of 2028-2030 to send a mission to Neptune. The probes to the ice giants must use the gravitational assistance of Jupiter to reduce the duration of the trip, but Jupiter and Neptune are in the correct position every twelve years or so (the time it takes for Jupiter to circle the Sun). The probe would reach Neptune in 2043, approximately (the exact duration would depend on the trajectory and launcher used). Regarding the harsh conclusions of the Mid-Term Review report of the Decadal Survey, in which it was concluded that none of the proposals of probes to the ice giants were scientifically adequate, the OPAG defends itself by saying that these were not solid designs, but proposals, especially with regard to scientific instrumentation (incidentally, also defends the reviled mission Europa Lander).

Possible trajectory of a probe to Neptune according to the OPAG (NASA).

Obviously, a probe to Neptune competes directly with the mission proposals to explore Enceladus, one of the fundamental objectives of researchers studying the ocean worlds, so it is difficult for NASA to approve a mission flagship to Neptune without having given the green light to a ship that studies this small moon of Saturn. The only way out, rather remote, would be to send a New Frontier type probe to Enceladus. On the other hand, the recent rapprochement between NASA and ESA to collaborate in the next mission to the ice giants can accelerate the project of a probe to Neptune. It is possible that the next Neptune probe carries several European instruments or, perhaps, a European atmospheric probe. If this mission is approved to be launched before 2031, something unlikely, we will not see Neptune up close until 2043 at the earliest. More than half a century after Voyager 2 unveiled its wonders for the first time. Honestly, the terms of the exploration of the outer solar system are somewhat depressing.

Neptune and Triton seen by Hubble (NASA / ESA).

References:

https://www.lpi.usra.edu/opag/meetings/sep2018/findings.pdf